For preparing ethylene polymers, generally known in the art is a process of copolymerizing ethylene with an .alpha.-olefin or polymerizing ethylene in the presence of a Ziegler catalyst. In this process, a high-temperature solution polymerization wherein polymerization is conducted in a hydrocarbon solvent at a temperature higher than the melting point of the resultant polymer is broadly utilized. However, if a polymer having a high molecular weight is intended to be obtained, a polymer concentration in the polymer solution must be lowered because the viscosity of the polymer solution becomes high with increase of the molecular weight. As a result, a problem of low productivity of a polymer takes place.
On the other hand, in the case of conducting the polymerization by a slurry polymerization process, there resides other problem. That is, the resultant polymer swells easily in a polymerization solvent. As a result, the concentration of the slurry can be hardly heightened and a long-term continuous polymerization operation can be difficultly made.
The present inventors have studied in the light of the problems associated with the prior art as mentioned above, and proposed a titanium catalyst component for ethylene polymerization as described in, for example, Japanese Patent Laid-Open Publication No. 195108/1985. This titanium catalyst component is excellent in handling property as a slurry and makes it possible to conduct polymerization operation in a high concentration of the slurry. In the titanium catalyst, more than 70% by weight of titanium atoms are reduced to trivalent state. By the use of this titanium catalyst component, an ethylene polymer having an excellent composition (copolymer) distribution can be prepared with a high polymerization activity.
As described above, by the use of the catalyst component for ethylene polymerization disclosed in Japanese Patent Laid-Open Publication No. 195108/1985, ethylene can be polymerized with a high polymerization activity, and moreover, an ethylene copolymer having a narrow composition distribution and excellent morphology can be obtained. However, now eagerly desired is a titanium catalyst component for ethylene polymerization by the use of which an ethylene polymer can be prepared with a high polymerization activity.
Further, Japanese Patent Publication No. 45404/1988 discloses a process for the preparation of a solid titanium catalyst component. In this process, the solid titanium catalyst component is prepared by contacting a magnesium aluminum complex which is obtained by the contact of an alcohol solution of halogen-containing magnesium with an organoaluminum compound, an electron donor having no active hydrogen and titanium tetrachloride to react with each other. The solid titanium catalyst component obtained in this process has a good particle size distribution, and hence polyolefin obtained by using a catalyst comprising this solid titanium catalyst component also has a good particle size distribution. However, the advent of a solid titanium catalyst component which has excellent catalytic activity for ethylene polymerization has been eagerly desired.
Furthermore, Japanese Patent Laid-Open Publication No. 159806/1982 discloses:
a solid titanium catalyst component prepared by bringing a product obtained by reacting a reaction product (a.sub.1) of halogen-containing magnesium and an alcohol with an organoaluminum compound into contact with a halogen-containing titanium compound, said solid titanium catalyst component having a molar ratio of alkoxy group and/or alcohol to titanium of not more than 0.25; PA1 a solid titanium catalyst component prepared by bringing a product obtained by reacting a reaction product (a.sub.1) of halogen-containing magnesium and an alcohol with an organoaluminum compound into contact with a halogen-containing titanium compound and subsequently bringing the resulting product into contact with an organoaluminum halide (halogenation agent), said solid titanium catalyst component having a molar ratio of alkoxy group and/or alcohol to titanium of not more than 0.9; and PA1 a solid titanium catalyst component obtained by bringing a product prepared by reacting a reaction product (a.sub.1) of halogen-containing magnesium and an alcohol with an organoaluminum compound into contact with an organoaluminum halide (halogenation agent), subsequently bringing the resulting product into contact with a halogen-containing titanium compound and further bringing the resulting product into contact with an organoaluminum halide (halogenation agent), said solid titanium catalyst component having a molar ratio of alkoxy group and/or alcohol to titanium of not more than 0.9. PA1 a solid magnesium aluminum complex obtained by contacting a solution formed from halogen-containing magnesium, an alcohol and a hydrocarbon solvent with organoaluminum, PA1 a tetravalent compound in a liquid state, and PA1 a vanadium compound, a zirconium compound or a hafnium compound. PA1 [A] a solid magnesium aluminum complex containing magnesium, halogen, aluminum and an alkoxy group and/or alcohol having at least 6 carbon atoms, said complex being obtained by contacting PA1 [B] a tetravalent titanium compound,
In such solid titanium catalyst components, a molar ratio Ti.sup.3+ /Ti.sup.4+ is in the range of 2.0 to 10, and most of the tetravalent titanium atoms are reduced into trivalent state. By the use of these catalyst components, ethylene can be polymerized with a high polymerization activity. However, further desired is the advent of a solid titanium catalyst component for ethylene polymerization which has a narrower particle size distribution and by the use of which ethylene can be polymerized with much higher polymerization activity.
Moreover, Japanese Patent Laid-Open Publication No. 91106/1992 discloses a solid titanium catalyst component obtained by contacting with each other:
In the comparative example of the above Japanese Patent Laid-Open Publication No. 91106/1992, an experiment wherein 2-ethylhexoxytitanium trichloride was used as the tetravalent titanium compound and no vanadium compound was used is shown.
In such solid titanium catalyst component as obtained above, most of titanium atoms are reduced into trivalent state, and the catalyst component is low in the activity for ethylene polymerization. Accordingly, a solid titanium catalyst component having a much higher activity for ethylene polymerization is now desired.